In wireless communications, orthogonal frequency division multiplexing (OFDM) may be used as a waveform structure. OFDM has many advantages, including the ease of implementation using fast Fourier transform (FFT) and inverse FFT (IFFT) and robustness against multi-path fading. However, OFDM has drawbacks, such as spectral inefficiency from cyclic-prefix (CP) and frequency guard bands.
OFDM-offset quadrature amplitude modulation (OQAM) is a multi-carrier transmission technique which uses time-frequency localized orthogonal prototype filters, such as root-raised cosine (RRC) and isotropic orthogonal transform algorithm (IOTA) pulse shapes. OFDM-OQAM has a superior power spectral density (PSD) side-lobe decay compared to OFDM. Hence, OFDM-OQAM may reduce the guard band overhead compared to OFDM. For example, in a long term evolution (LTE) system, the overhead may be reduced by 10%, in addition to the gain in spectral efficiency from CP removal. However, OFDM-OQAM incurs an overhead from transmission times of tails at both ends of a transmission burst. OFDM-OQAM has an overhead due to the T/2 time offset between the OQAM symbols, where T is the symbol duration. The total overhead duration is equal to the length of the prototype filter, minus T/2. The length of the prototype filter may be at least 4T to preserve an acceptable inter-symbol interference (ISI) and inter-carrier interference (ICI). For a burst length of 28 OQAM symbols, this is 7/28=25% overhead in time.
Circular convolution has been used in OFDM-OQAM to remove the overhead associated with the pulse tails. It was first proposed in generalized frequency division multiplexing (GFDM) and was later extended to OFDM-OQAM. All the circular convolution based signaling uses the assumption that the channel remains constant in the period for circular convolution. However, in a cellular system, this assumption is often not true. When this happens, the orthogonality between the signal pulses is lost.